Something all hikers might want to remember when traveling the hundreds of trails all around Southern California.

Just the slightest brush with poison oak will have the hiker in “itches” with red, oozing bumps for up to 10 days.

Los Angeles Park Ranger Ernie Ybarra, “Ranger Ernie,” is based at Griffith Park and often includes poison oak awareness on the free public hikes he leads.

“Poison oak can be found on all trails and is infectious with the slightest touch — even when dead,” said Ranger Ernie. “The best way to avoid poison oak is to stay on the trail.”

Ranger Ernie explained the main way to identify poison oak is by its leaves that grow in threes. An easy way to remember to stay clear is by remembering his simple rhyme: “Leaves of three, let it be.”

He went on to say that poison oak leaves start out a bright, shiny green in spring and summer and then turn a red/orange in the fall before they go brown and fall off in winter.

“Poison oak is toxic in all stages; it’s basically a flesh-eating disease,” Ranger Ernie said. “But if you do get it — don’t scratch.”

Ranger Ernie’s advice is universal and touted by those in the know, like Alex Fauce, a teacher and avid hiker. Fauce spent the summer of 2012 teaching science to elementary school students at a Big Bear sleep away camp.

“We were always very clear with the kids to ‘stay on the trail,’ and if they have any questions about the foliage they see, come get one of us,” said Fauce, a 27-year-old Upland resident. “Coming in contact with poison oak can sometimes just come down to negligence. You have to pay attention to your surroundings.”

Fauce, who enjoys several mountain activities, has never been affected by poison oak.

“But some of my friends have and I’m pretty sure they got it when we went off trail,” he chuckled.

After decades of hiking, Ann Olander, 72, of Rancho Cucamonga, has never contracted poison oak, but her son, Doug, has.

“(Doug) got poison oak on his jeans and shoes, resulting in one small boil, but the next time he put those jeans back on he said, ‘It felt like ants crawling over my legs,’ ” recounted Olander. “He went to the clinic where they gave him oral cortical steroids to help reduce the inflammation. When he came home, it looked to me like he had boils all over his legs plus much of his body.”

Olander, along with her husband Farley, authored the “Call of the Mountains,” a photo-essay book spotlighting the beauty of Southern California’s three highest mountain ranges: San Jacinto, San Bernardino and San Gabriel.

Olander explained poison oak’s allergic reaction is caused by urushiol, an oil in the plant that can be contracted three ways: direct, indirect or airborne contact.

Her son and his friends went off trail to explore a rock climbing spot when he came in contact with the plant.

“He thought he was safe because his skin was covered; but the poison oak wasn’t on his skin, it was on his jeans,” she said. A reminder that all clothing worn during hiking should be carefully stowed and washed before being worn again.

Another poison-oak pitfall are trails with switchbacks. On the way down, hikers sometimes cut across the switchbacks, with itchy results.

“It’s tempting to bypass the switchbacks, going straight down off trail and through the bush. But that’s exactly where poison oak is likely found,” she said. “So stay on the trail. In all my years of hiking, I’ve never seen it on a trail.”